Modern aircraft are manufactured from numerous panels and other parts that are fastened together with rivets, screws, bolts, and other permanent fasteners. To aid in assembly, parts are typically held together with temporary clamps and fixtures until permanent fasteners can be installed. Parts that incorporate sealant on the mating surfaces require that the temporary clamps exert sufficient force to squeeze excessive sealant from the joint while pulling parts together before the sealant fully cures. For example, some heavy structures fastened with five-sixteenths of an inch size permanent fasteners, require in excess of 500 pounds clamp load to squeeze the sealant out to an acceptable thickness and hold the components together. Other applications, such as in wing-to-body joints, require upwards of 1500 pounds with five-sixteenths of an inch size fasteners to temporarily secure components. The clamp load requirements for other sizes are generally proportional to the cross-sectional area of the basic fastener diameter.
Blind hole clamps are desirable for airframe assembly, because their installation and removal can be more easily automated than the installation and removal of conventional bolts and nuts. However, some existing blind hole clamps cause surface damage either during the formation of the blind head or because the blind head is ineffective in distributing the clamp load over the surface.
Oftentimes, one or more work pieces are joined with clamps to maintain orientation during an autoclave curing cycle. Threaded-type reusable blind clamps are capable of high clamp loads, but lack the smooth shank needed to avoid clogging with resin as the parts are cured. As a result, the clamps are difficult to remove and may damage the work pieces upon removal. Furthermore, rework of the holes to remove excess cured shim or sealant may be required which, in addition to requiring additional time and materials, may also damage the work pieces. Some blind tack rivets have the required smooth shank but are incapable of imparting sufficient clamp load to maintain parts in the required orientation.
In addition, threaded temporary blind clamps are easily clogged with sealant and resins, making removal from assemblies difficult and necessitating cleaning and rework of the blind clamps before they may be reused. Another problem with threaded temporary fasteners is that they protrude above the accessible panel surface by a relatively large amount. Accordingly, robotic assembly equipment must retract or back away from the work pieces to avoid collisions with installed clamps. As a result, installation of threaded temporary fasteners requires additional time for robotic assembly equipment to traverse from one location to another.
Finally, the clamping capability of many threaded temporary clamps is limited, because the blind head may be discontinuous and high clamp loads result in surface damage to the work pieces. Temporary blind tack rivets have a low profile but must be removed by drilling through the manufactured head. Unfortunately, drilling through the rivet head can also damage panel surfaces. Another issue, for example, is that the rivet can spin in the hole during the drilling operation, halting the advance of the drill bit through the tack rivet and prolonging the removal cycle time. Tack rivets also have very low clamp loads and produce a small blind-side upset that is not suitable for use in laminated composite panels.